The present invention relates to a fuel pump comprising a plurality of plungers and a camshaft having a plurality of drive cams. Each of the drive cams corresponds to one of the plungers, and the camshaft with the drive cams engages the plungers in reciprocal movement by rotating the camshaft. The present invention also relates to a fuel feeding device that employs this fuel pump.
A fuel feeding device adopting the so-called common rail system comprises a fuel pump, a common rail in which high-pressure fuel force-fed from the fuel pump is stored, and fuel injection valves each provided in correspondence to one of the cylinders of an internal combustion engine to enable a fuel feed of the high-pressure fuel stored in the common rail to enter the cylinders. The fuel pump normally includes two plungers, and these plungers are caused to move reciprocally by drive cams provided as separate units at the camshaft to supply pressurized fuel to the common rail. In a standard common rail system through which fuel is fed to, for instance, a six-cylinder engine by employing two plungers, three cam lobes are provided over constant intervals at each of the drive cams that drive the individual plungers. The phases of the drive cams are offset from each other by 60xc2x0 so as to achieve six injections by allowing the plungers alternately to make three reciprocal movements as the camshaft rotates over 360xc2x0 once, as disclosed in Patent Official Gazette No. 2797745.
As is understood from the cam lift characteristics and the shape of the cams described in the publication, a fuel pump such as the one described above normally assumes the shape shown in FIG. 7(A). Namely, cam lobes formed at the individual drive cams xcex1 and xcex2 are each formed to achieve a shape having portions used for a forward stroke. (i.e., movement from bottom dead center to top dead center) and a backward stroke (i.e., movement from top dead center to bottom dead center) of the corresponding plunger, and the shapes are symmetrical with respect to each other so that each drive cam takes on a triangular shape overall.
As a result, the lift characteristics manifesting at each of the plungers driven by the drive cams xcex1 and xcex2 during the forward stroke, in which the plunger travels from the bottom dead center to the top dead center, (i.e., away from the center of rotation of the drive cams) and the lift characteristics manifesting at the same plunger during the backward stroke, in which the plunger travels from the top dead center to the bottom dead center (i.e., toward the center of rotation of the drive cams) are symmetrical. In addition, the lift characteristics of one of the plungers manifest as sine waves whose phase is offset by 60xc2x0 from the sine waves representing the lift characteristics of the other plunger, as shown in FIG. 7(B). Since one of the plungers first ascends from the bottom dead center to the top dead center and completes an injection, and then the other plunger starts ascending from the bottom dead center in the structure of the related art described above, the geometric injection rate (GIR) achieves characteristics whereby the value of the geometric injection rate continuously changes between 0 and the peak value every 60xc2x0 of the cam rotating angle (cam angle), as shown in FIG. 7(C). Since the cam speed and the drive torque are roughly in proportion to the characteristics of the geometric injection rate, the plunger lift speed (i.e., the cam speed) and the drive torque, too, manifest characteristics whereby they fluctuate in a similar manner.
While a fuel pump having the symmetrical cams described above is normally utilized in an injection pump employed in a common rail system in the related art, a number of problems discussed below arise with regard to a fuel feeding device that employs such a fuel pump.
Namely, if the drive cams illustrated in FIG. 7(A) are utilized, the geometric injection rate (GIR) constantly fluctuates between 0 and the peak value over every 60xc2x0, causing a significant fluctuation in the pressure within the common rail. In addition, since the plunger must be lifted to the highest lift position while rotating the drive cam by 60xc2x0, the fluctuation in the cam speed, too, is bound to be great, which, in turn, requires a large drive torque.
Furthermore, since the plunger must be lifted to the highest lift position over a small cam rotating angle (60xc2x0 in the example described above), it becomes necessary to form the cam nose with a small radius of curvature. This results in a large force being applied onto the cam surface while lifting the plunger, so that the surface pressure becomes a problem.
When the fuel pump in the prior art with the problems discussed above is utilized in a common rail system, the range of application in engines becomes limited, and the durability of the overall system is lowered.
Namely, while the pressure-withstanding performance of the product is normally designed to provide an ample margin to comfortably tolerate even the upper limit of the pressure fluctuation to maximize the service life of the product, the pressure-withstanding level of the overall system, including the fuel injection valves, the common rail, the piping connecting the fuel pump with the common rail, and the piping connecting the common rail with the fuel injection valves must be extremely high if the fluctuation of the pressure of the fuel let out from the fuel pump is great. For this reason, a significant fluctuation in the pressure gives rise to problems in that the weight of the product is bound to increase since the components need greater wall thickness, and in that the structure of the product becomes more complicated in order to achieve better pressure-withstanding performance.
In addition, since ignitions normally occur over irregular intervals in the engine combustion chamber of an engine having 10 or more cylinders, the timing with which the fuel is injection into the engine and the timing with which the fuel is fed from the fuel pump to the common rail cannot match each other if a fuel pump having drive cams corresponding to 6 cylinders is used as a replacement in conjunction with such an engine in which ignitions occur over irregular intervals. As a result, if the injection rate of the fuel pump fluctuates greatly, as illustrated in FIG. 7(C), inconsistency occurs between the injection characteristics manifesting as the fuel is injected from a fuel injection valve while the injection rate is low and the injection characteristics manifesting as the fuel is injected while the injection rate is high. For this reason, the injection pump in the prior art and a fuel feeding device that utilizes the injection pump cannot be employed in conjunction with engines in which ignitions occur over irregular intervals.
It is conceivable to increase the number of cam lobes in correspondence to a larger number of cylinders provided in the engine, or to increase the numbers of plungers and drive cams if the first option is not feasible, in order to solve the problem. However, it is difficult to secure a sufficient angle range for forming each cam lobe when the number of cam lobes formed at the drive cams is increased. Accordingly, it becomes necessary to increase the diameter of the drive cams to achieve the required lift quantity, or to increase the wall thickness of the drive cams to withstand the pressure applied to the cam surfaces. Thus, the dimension of the camshaft along the radial direction increases if the diameter of the drive cams is increased, or the dimension of the camshaft along the axial direction increases if the wall thickness of the drive cams is increased. Furthermore, the dimension of the camshaft along the axial direction increases instead when the numbers of plungers and drive cams are increased.
Moreover, when the drive cams in the related art described above are utilized, the drive torque constantly fluctuates between 0 and the peak valve. As a result, the load on the drive system and the noise occurring in the system are bound to be significant. In addition, the product must be designed by adopting a structure with ample margin for drive torque fluctuations, so that the drive system must be thick and heavy to tolerate such drive torque fluctuations.
Accordingly, an object of the present invention is to provide a fuel pump having drive cams with which the problems discussed above can be solved and a fuel feeding device utilizing the fuel pump.
In order to achieve the object described above, the fuel pump according to the present invention comprises a plurality of plungers and a camshaft having a plurality of drive cams each corresponding to one of the plurality of plungers with a motive force applied from the outside used to rotate the camshaft so that the plurality of plungers engage the corresponding drive cams in reciprocal movement and the fuel pressurized and force-fed during a forward stroke of each of the plungers. All or some of the plurality of drive cams are set by offsetting their phases from one another, and each of the drive cams includes asymmetrical cam lobes each formed so as to reduce the extent of displacement of the corresponding plunger relative to a unit cam rotating angle during the forward stroke (i.e., motion from top dead center to bottom dead center) compared to the extent of displacement occurring during the backward stroke of the plunger.
In addition, the fuel feeding device according to the present invention has a fuel pump, a common rail in which high-pressure fuel force-fed from the fuel pump is stored, and fuel injection valves each provided in correspondence to one of the cylinders of an internal combustion engine which allow the high-pressure fuel stored in the common rail to be fed. The fuel pump comprises a plurality of plungers and a camshaft having a plurality of drive cams each provided in correspondence to one of the plurality of plungers. A motive force is applied from the outside and used to rotate the camshaft to engage the plurality of plungers in reciprocal movement with the corresponding drive cams and the fuel pressurized and force-fed during a forward stroke of each of the plungers. All or some of the plurality of drive cams are set by offsetting their phases from one another, and each of the drive cams includes asymmetrical cam lobes each formed so as to reduce the extent of displacement of the corresponding plunger relative to a unit cam rotating angle during the forward stroke compared to the extent of displacement occurring during the backward stroke of the plunger.
Thus, by utilizing the fuel pump having the drive cams described above, in which the extent of the displacement of each plunger per unit of cam rotating angle (i.e., the extent of change in the lift) is reduced during the forward stroke of the plunger than the extent of plunger displacement occurring during the backward stroke of the plunger, the plunger can be lifted more slowly compared to the prior art during the forward stroke, and the plunger can also be reset quickly during the backward stroke even if the number of cam lobes provided at each drive cam is the same as that in the related art. As a result, the geometric injection rate of the fuel pump and the maximum drive torque, which is in proportion to the geometric injection rate, can be set smaller than in a structure utilizing the symmetrical cams in the related art.
Furthermore, even if the cam rotating angle allocated in correspondence to each cam lobe is small, the cam lobes assume an asymmetrical shape whereby the extent of plunger displacement relative to the unit cam rotating angle is smaller during the forward stroke than in the backward stroke. As a result, it is possible to achieve a larger radius of curvature at the cam nose than in the prior art.
It is desirable to form the cam lobes at the drive cams so that they assume a concave shape over the areas corresponding to the backward stroke of the plungers.
By forming the drive cams in such a shape, the angle range of the portion of each of the drive cams required for the backward stroke can be further reduced so as to assure a larger angle range of the portion of each of the drive cams allocated for the forward stroke while ensuring that the plungers move along the backward direction quickly. While it goes without saying that the portion of each cam lobe corresponding to the backward stroke should be formed over an angle range (i.e., an angle of an arc spanning a portion of each drive cam) in which jumping of the plunger or the tappet provided between the plunger and the cam lobe is prevented, the shape described above is particularly effective when a large number of cam lobes are formed with a small angle range allocated for each cam lobe. Thus, it is necessary to lift the plungers slowly by maximizing the angle range corresponding to the forward stroke.
It is desirable that the asymmetrical cam lobes be formed at the plurality of drive cams so that the injection rates of the individual plungers achieve a roughly constant total over a given cam rotating angle.
The cam lobes at the drive cams are formed in an asymmetrical shape whereby the extent of change in the plunger lift per unit cam rotating angle is smaller during the forward stroke than the extent of change manifesting during the backward stroke of the plungers so as to ensure that an almost constant total is achieved by the injection rates of the individual plungers over a given cam rotating angle. Therefore, it is no longer necessary to synchronize the reciprocal movement of the plungers with engine ignitions even when the fuel pump is utilized in conjunction with an engine in which ignitions occur over irregular intervals. In other words, when this fuel pump is utilized in a common rail system, the quantity of fuel fed to the common rail hardly fluctuates and, as a result, the extent of pressure fluctuation occurring within the common rail can be lessened. Thus, no significant disruption occurs in the injection characteristics even if the system is employed in conjunction with an engine in which ignitions occur over irregular intervals.